CREATE CLUSTER

Purpose

Use the CREATE CLUSTER statement to create a cluster. A cluster is a schema object that contains data from one or more tables.

• An indexed cluster must contain more than one table, and all of the tables in the cluster have one or more columns in common. Oracle Database stores together all the rows from all the tables that share the same cluster key.

• In a hash cluster, which can contain one or more tables, Oracle Database stores together rows that have the same hash key value.

For information on existing clusters, query the USER_CLUSTERS, ALL_CLUSTERS, and DBA_CLUSTERS data dictionary views.

See Also:

• Oracle Database Concepts for general information on clusters

• Oracle Database SQL Tuning Guide for suggestions on when to use clusters

• Oracle Database Reference for information on the data dictionary views

Prerequisites

To create a cluster in your own schema, you must have CREATE CLUSTER system privilege. To create a cluster in another user's schema, you must have CREATE ANY CLUSTER system privilege. Also, the owner of the schema to contain the cluster must have either space quota on the tablespace containing the cluster or the UNLIMITED TABLESPACE system privilege.

Oracle Database does not automatically create an index for a cluster when the cluster is initially created. Data manipulation language (DML) statements cannot be issued against cluster tables in an indexed cluster until you create a cluster index with a CREATE INDEX statement.

Syntax

create_cluster::=

Description of the illustration create_cluster.eps

(physical_attributes_clause::=, size_clause::=, cluster_range_partitions::=)

physical_attributes_clause::=

Description of the illustration physical_attributes_clause.eps

(storage_clause::=)

parallel_clause::=

Description of the illustration parallel_clause.eps

cluster_range_partitions::=

Description of the illustration cluster_range_partitions.eps

(range_values_clause::=, table_partition_description::=)

Semantics

schema

Specify the schema to contain the cluster. If you omit schema, then Oracle Database creates the cluster in your current schema.

cluster

Specify is the name of the cluster to be created. The name must satisfy the requirements listed in "Database Object Naming Rules".

After you create a cluster, you add tables to it. A cluster can contain a maximum of 32 tables. Object tables and tables containing LOB columns or columns of the Any* Oracle-supplied types cannot be part of a cluster. After you create a cluster and add tables to it, the cluster is transparent. You can access clustered tables with SQL statements just as you can access nonclustered tables.

See Also:

CREATE TABLE for information on adding tables to a cluster, "Creating a Cluster: Example", and "Adding Tables to a Cluster: Example"

column

Specify one or more names of columns in the cluster key. You can specify up to 16 cluster key columns. These columns must correspond in both data type and size to columns in each of the clustered tables, although they need not correspond in name.

You cannot specify integrity constraints as part of the definition of a cluster key column. Instead, you can associate integrity constraints with the tables that belong to the cluster.

See Also:

"Cluster Keys: Example"

datatype

Specify the data type of each cluster key column.

Restrictions on Cluster Data Types

Cluster data types are subject to the following restrictions:

• You cannot specify a cluster key column of data type LONG, LONG RAW, REF, nested table, varray, BLOB, CLOB, BFILE, the Any* Oracle-supplied types, or user-defined object type.

• You can specify a column of type ROWID, but Oracle Database does not guarantee that the values in such columns are valid rowids.

See Also:

"Data Types" for information on data types

COLLATE

Use this clause to specify the data-bound collation for character data type columns in the cluster key.

For column_collation_name, specify the collation as follows:

• When creating an indexed cluster or a sorted hash cluster, you can specify one of the following collations: BINARY, USING_NLS_COMP, USING_NLS_SORT, or USING_NLS_SORT_CS.

• When creating a hash cluster that is not sorted, you can specify any valid named collation or pseudo-collation.

If you omit this clause, then columns in the cluster key inherit the effective schema default collation of the schema containing the cluster. Refer to the DEFAULT_COLLATION clause of ALTER SESSION for more information on the effective schema default collation.

The collations of cluster key columns must match the collations of the corresponding columns in the tables created in the cluster.

You can specify the COLLATE clause only if the COMPATIBLE initialization parameter is set to 12.2 or greater, and the MAX_STRING_SIZE initialization parameter is set to EXTENDED.

To change the collation of a cluster key column, you must recreate the cluster.

SORT

The SORT keyword is valid only if you are creating a hash cluster. Table rows are hashed into buckets on cluster key columns without SORT, and then sorted in each bucket on the columns with this clause. This may improve response time during subsequent operations on the clustered data.

All columns without the SORT clause must come before all columns with the SORT clause in the CREATE CLUSTER statement.

Restriction on Sorted Hash Clusters

Row dependency is not supported for sorted hash clusters.

See Also:

• See "HASHKEYS Clause" for information on creating a hash cluster.

• Managing Hash Clusters for more information.

physical_attributes_clause

The physical_attributes_clause lets you specify the storage characteristics of the cluster. Each table in the cluster uses these storage characteristics as well. If you do not specify values for these parameters, then Oracle Database uses the following defaults:

• PCTFREE: 10

• PCTUSED: 40

• INITRANS: 2 or the default value of the tablespace to contain the cluster, whichever is greater

  See Also:

  physical_attributes_clause and storage_clause for a complete description of these clauses

SIZE

Specify the amount of space in bytes reserved to store all rows with the same cluster key value or the same hash value. This space determines the maximum number of cluster or hash values stored in a data block. If SIZE is not a divisor of the data block size, then Oracle Database uses the next largest divisor. If SIZE is larger than the data block size, then the database uses the operating system block size, reserving at least one data block for each cluster or hash value.

The database also considers the length of the cluster key when determining how much space to reserve for the rows having a cluster key value. Larger cluster keys require larger sizes. To see the actual size, query the KEY_SIZE column of the USER_CLUSTERS data dictionary view. (This value does not apply to hash clusters, because hash values are not actually stored in the cluster.)

If you omit this parameter, then the database reserves one data block for each cluster key value or hash value.

TABLESPACE

Specify the tablespace in which the cluster is to be created.

INDEX Clause

Specify INDEX to create an indexed cluster. In an indexed cluster, Oracle Database stores together rows having the same cluster key value. Each distinct cluster key value is stored only once in each data block, regardless of the number of tables and rows in which it occurs. If you specify neither INDEX nor HASHKEYS, then Oracle Database creates an indexed cluster by default.

After you create an indexed cluster, you must create an index on the cluster key before you can issue any data manipulation language (DML) statements against a table in the cluster. This index is called the cluster index.

You cannot create a cluster index for a hash cluster, and you need not create an index on a hash cluster key.

See Also:

CREATE INDEX for information on creating a cluster index and Oracle Database Concepts for general information in indexed clusters

HASHKEYS Clause

Specify the HASHKEYS clause to create a hash cluster and specify the number of hash values for the hash cluster. In a hash cluster, Oracle Database stores together rows that have the same hash key value. The hash value for a row is the value returned by the hash function of the cluster.

Oracle Database rounds up the HASHKEYS value to the nearest prime number to obtain the actual number of hash values. The minimum value for this parameter is 2. If you omit both the INDEX clause and the HASHKEYS parameter, then the database creates an indexed cluster by default.

When you create a hash cluster, the database immediately allocates space for the cluster based on the values of the SIZE and HASHKEYS parameters.

See Also:

Oracle Database Concepts for more information on how Oracle Database allocates space for clusters and "Hash Clusters: Examples"

SINGLE TABLE

SINGLE TABLE indicates that the cluster is a type of hash cluster containing only one table. This clause can provide faster access to rows in the table.

Restriction on Single-table Clusters

Only one table can be present in the cluster at a time. However, you can drop the table and create a different table in the same cluster.

See Also:

"Single-Table Hash Clusters: Example"

HASH IS expr

Specify an expression to be used as the hash function for the hash cluster. The expression:

• Must evaluate to a positive value

• Must contain at least one column, with referenced columns of any data type as long as the entire expression evaluates to a number of scale 0. For example: number_column * LENGTH(varchar2_column)

• Cannot reference user-defined PL/SQL functions

• Cannot reference the pseudocolumns LEVEL or ROWNUM

• Cannot reference the user-related functions USERENV, UID, or USER or the datetime functions CURRENT_DATE, CURRENT_TIMESTAMP, DBTIMEZONE, EXTRACT (datetime), FROM_TZ, LOCALTIMESTAMP, NUMTODSINTERVAL, NUMTOYMINTERVAL, SESSIONTIMEZONE, SYSDATE, SYSTIMESTAMP, TO_DSINTERVAL, TO_TIMESTAMP, TO_DATE, TO_TIMESTAMP_TZ, TO_YMINTERVAL, and TZ_OFFSET.

• Cannot evaluate to a constant

• Cannot be a scalar subquery expression

• Cannot contain columns qualified with a schema or object name (other than the cluster name)

If you omit the HASH IS clause, then Oracle Database uses an internal hash function for the hash cluster.

For information on existing hash functions, query the USER_, ALL_, and DBA_CLUSTER_HASH_EXPRESSIONS data dictionary tables.

The cluster key of a hash column can have one or more columns of any data type. Hash clusters with composite cluster keys or cluster keys made up of noninteger columns must use the internal hash function.

See Also:

Oracle Database Reference for information on the data dictionary views

parallel_clause

The parallel_clause lets you parallelize the creation of the cluster.

For complete information on this clause, refer to parallel_clause in the documentation on CREATE TABLE.

NOROWDEPENDENCIES | ROWDEPENDENCIES

This clause has the same behavior for a cluster that it has for a table. Refer to "NOROWDEPENDENCIES | ROWDEPENDENCIES" in CREATE TABLE for information.

CACHE | NOCACHE

CACHE

Specify CACHE if you want the blocks retrieved for this cluster to be placed at the most recently used end of the least recently used (LRU) list in the buffer cache when a full table scan is performed. This clause is useful for small lookup tables.

NOCACHE

Specify NOCACHE if you want the blocks retrieved for this cluster to be placed at the least recently used end of the LRU list in the buffer cache when a full table scan is performed. This is the default behavior.

NOCACHE has no effect on clusters for which you specify KEEP in the storage_clause.

cluster_range_partitions

Specify the cluster_range_partitions clause to create a range-partitioned hash cluster. If you specify this clause, then you must also specify the HASHKEYS clause.

Use the cluster_range_partitions clause to partition the cluster on ranges of values from the column list. When you add a table to a range-partitioned hash cluster, it is automatically partitioned on the same columns, with the same number of partitions, and on the same partition bounds as the cluster. Oracle Database assigns system-generated names to the table partitions.

The cluster_range_partitions clause has the same semantics as the range_partitions clause of CREATE TABLE, except that here you cannot specify the INTERVAL clause. For complete information, refer to range_partitions in the documentation on CREATE TABLE.

See Also:

"Range-Partitioned Hash Clusters: Example"

Examples

Creating a Cluster: Example

The following statement creates a cluster named personnel with the cluster key column department, a cluster size of 512 bytes, and storage parameter values:

CREATE CLUSTER personnel
   (department NUMBER(4))
SIZE 512 
STORAGE (initial 100K next 50K);

Cluster Keys: Example

The following statement creates the cluster index on the cluster key of personnel:

CREATE INDEX idx_personnel ON CLUSTER personnel;

After creating the cluster index, you can add tables to the index and perform DML operations on those tables.

Adding Tables to a Cluster: Example

The following statements create some departmental tables from the sample hr.employees table and add them to the personnel cluster created in the earlier example:

CREATE TABLE dept_10
   CLUSTER personnel (department_id)
   AS SELECT * FROM employees WHERE department_id = 10;

CREATE TABLE dept_20
   CLUSTER personnel (department_id)
   AS SELECT * FROM employees WHERE department_id = 20;

Hash Clusters: Examples

The following statement creates a hash cluster named language with the cluster key column cust_language, a maximum of 10 hash key values, each of which is allocated 512 bytes, and storage parameter values:

CREATE CLUSTER language (cust_language VARCHAR2(3))
   SIZE 512 HASHKEYS 10
   STORAGE (INITIAL 100k next 50k);

Because the preceding statement omits the HASH IS clause, Oracle Database uses the internal hash function for the cluster.

The following statement creates a hash cluster named address with the cluster key made up of the columns postal_code and country_id, and uses a SQL expression containing these columns for the hash function:

CREATE CLUSTER address
   (postal_code NUMBER, country_id CHAR(2))
   HASHKEYS 20
   HASH IS MOD(postal_code + country_id, 101);

Single-Table Hash Clusters: Example

The following statement creates a single-table hash cluster named cust_orders with the cluster key customer_id and a maximum of 100 hash key values, each of which is allocated 512 bytes:

CREATE CLUSTER cust_orders (customer_id NUMBER(6))
   SIZE 512 SINGLE TABLE HASHKEYS 100;

Range-Partitioned Hash Clusters: Example

The following statement creates a range-partitioned hash cluster named sales with five range partitions based on the amount sold. The cluster key is made up of the columns amount_sold and prod_id. The cluster uses the hash function (amount_sold * 10 + prod_id) and has a maximum of 100000 hash key values, each of which is allocated 300 bytes.

CREATE CLUSTER sales (amount_sold NUMBER, prod_id NUMBER)
  HASHKEYS 100000
  HASH IS (amount_sold * 10 + prod_id)
  SIZE 300
  TABLESPACE example
  PARTITION BY RANGE (amount_sold)
    (PARTITION p1 VALUES LESS THAN (2001),
     PARTITION p2 VALUES LESS THAN (4001),
     PARTITION p3 VALUES LESS THAN (6001),
     PARTITION p4 VALUES LESS THAN (8001),
     PARTITION p5 VALUES LESS THAN (MAXVALUE));